1. Field of the Invention
The present invention relates to a liquid-crystal display device of active matrix type, and more particularly to an active matrix type liquid-crystal display device of IPS (In-Plane Switching) mode (horizontal electric field mode).
2. Description of the Related Art
There has been known a liquid-crystal display device of active matrix type employing active elements such as thin film transistors (TFTs). The active matrix type liquid-crystal display device is capable of heightening a pixel density, and it is small in size and light in weight and is of low dissipation power, so that the monitor of a personal computer, the display panel of a liquid-crystal television set, and the like products have been developed as substitutes for CRTs. In particular, a technique in which the active layer of each TFT is formed of a film of crystalline semiconductor typified by polycrystal silicon, permits the switching TFTs of pixel portions (hereinbelow, expressed as “pixel TFTs”) and driver circuits to be formed on an identical substrate, and it is regarded as a technique which contributes to reducing the size and lightening the weight of the liquid-crystal display device.
The liquid-crystal display device is such that a liquid crystal is enclosed between a pair of substrates, and that liquid crystal molecules are oriented by electric fields which are substantially perpendicular to the planes of the substrates and which are applied across the pixel electrodes (individual electrodes) of one of the substrates and the counter electrode (common electrode) of the other substrate. Such a method of driving the liquid crystal, however, has had the disadvantage of a narrow angle of vision due to which, even in a normal display state as viewed in a direction perpendicular to the substrate planes, an image viewed obliquely becomes an unclear one involving changes in color tones.
An expedient for overcoming the disadvantage is the IPS mode. This mode features that pixel electrodes and a common wiring line are both formed on one substrate, and that electric fields are switched in a horizontal direction. Thus, liquid crystal molecules have their orientations controlled in a direction substantially parallel to the plane of the substrate without rising up. The angle of vision can be widened by the operating principle.
FIG. 5 shows an example of a pixel structure in an active matrix type liquid-crystal display device of IPS mode in the prior art. Referring to the figure, numeral 301 designates a gate wiring line, numeral 302 the semiconductor film of a TFT, numeral 303 a common wiring line, each of numerals 304 and 308 a signal wiring line (source wiring line), numeral 305 a pixel electrode, numeral 307 a counter electrode, and numeral 306 a storage capacitor portion.
With the pixel structure, gaps exist between the counter electrode 307 and the signal wiring lines 304, 308, and a liquid crystal cannot be driven in accordance with an image signal at the parts of the gaps as well as the signal wiring lines 304, 308, so that the problem of light leakages occurs. Light shield films need to be formed at the parts in order to prevent the light leakage, but they result in lowering the aperture ratio of a pixel portion. With the pixel structure as shown in FIG. 5, the aperture ratio is limited to at most 30-40% or so, and the intensity of back light needs to be heightened in order to ensure brightness. However, when the intensity of the back light is heightened, increase in dissipation power is incurred, and moreover, the lifetime of the back light itself might be shortened.